1. Field of the Invention
This invention relates to the effective and economical separation of ammonia from gaseous mixture containing ammonia and hydrogen cyanide such as is produced in the Andrussow process for hydrogen cyanide.
2. The Prior Art
The Andrussow hydrogen cyanide process, as described in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 6, pages 577 to 579 (1965), involves passing a mixture of methane or natural gas, ammonia and air over a platinum catalyst at 1000.degree. to 1200.degree. C to produce an effluent containing hydrogen cyanide, water and unreacted ammonia.
Most Andrussow-process HCN plants employ sulfuric acid to scrub the ammonia out of the HCN-containing gas leaving the reactor. The ammonia reacts with the sulfuric acid to form a reasonably concentrated ammonium sulfate solution. The solution can be processed to recover solid ammonium sulfate for sale as fertilizer.
In some instances this practice is being continued even though the selling price of the ammonium sulfate is less than its production cost. In other instances, the ammonium sulfate is thermally decomposed and the SO.sub.2 converted back to sulfuric acid because of the low value of the ammonium sulfate.
Other Andrussow plants employ removal of the ammonia with a solution of ammonium hydrogen phosphate such as described in Carlson, U.S. Pat. No. 2,797,148 and Carlson et al., U.S. Pat. No. 3,718,731, both incorporated by reference herein.
The ammonium hydrogen phosphate system has several drawbacks. For example:
1. The solution containing a mixture of monoammonium and diammonium phosphate, even when conditions are optimal, is a relatively poor absorbent for NH.sub.3 so that there must be a large number of transfer stages in the absorber. This means a tall absorber is required equipped with an appreciable number of trays. Not only does this requirement necessitate a substantial capital investment, it also means energy has to be continuously supplied to overcome the pressure drop experienced by the gas as it passes upward counter-current to the absorbent liquor in the absorption tower. In addition, the absorbent leaves a small amount of ammonia in the gas.
2. The trace of ammonia left in the gas must be removed in a second scrubber. Sulfuric acid is fed to the second scrubber. The resulting solution of sulfuric acid and ammonium sulfate is bled from the process. Prior to being discarded, the bleed ammonium sulfate solution is treated to destroy its ammonia content because current effluent regulations prohibit the discharge of more than modest amounts of Kjeldahl nitrogen.
3. Because the "phosphate" solution is a relatively poor absorbent for ammonia and its absorbent power diminishes with increase in temperature, the contacting of the absorbent liquor and ammonia-containing gas has to be carried out at a relatively low temperature. Because the solution leaving the ammonia absorber is relatively low in temperature with a pH close to 8, some HCN also dissolves in the absorbent. This HCN must be separated and recovered from the solution prior to its regeneration, further increasing the required capital investment and the cost of ammonia recovery.
4. The ammonia-rich phosphate solution which has an NH.sub.4.sup.+ /PO.sub.4 .tbd. ratio close to 1.8 is regenerated by protracted fractional distillation until the NH.sub.4.sup.+ /PO.sub.4 .tbd. ratio is reduced to about 1.3. Approximately 25 pounds of steam are expended for each pound of recovered ammonia. To minimize the amount of steam needed, a tall, well-designed stripper has to be used, further increasing the required capital investment. To maintain the phosphate solution at the correct concentration nearly all the water vaporized is condensed and returned as reflux. This requires a proportionately large condenser and the circulation of a relatively large volume of cooling water.
5. The vapor leaving the steam stripper is condensed to form an aqueous ammonia solution. This solution must be fractionally distilled, preferably under pressure, to recover a very concentrated ammonia fraction suitable for recycling to the reactor in which the HCN is formed.
Obviously, the ammonium phosphate process, with all of its drawbacks, in some circumstances is preferably to expending money on an acid such as sulfuric acid and then receiving little or no return from the ammonium compound that is formed.
It is equally obvious that an improved process over previous proposals would be a scheme by which the NH.sub.3 would be efficiently separated from the HCN with a minimum of capital investment and total operating cost, including such items as the cost of circulating absorbent solution, the cost of supplying energy to cause the HCN-containing gas to flow through the ammonia absorption equipment, the cost of cooling water, the cost of removing HCN from the ammonium salt-containing absorption liquor, and the like.
Of overriding importance is the use of an absorption process that minimizes the formation of HCN polymers termed azulmic acid, Such polymers tend to form when HCN is allowed to stay in contact with aqueous solutions at elevated temperatures in the absence of acidic conditions.
In spite of the use of various precautions, all HCN production plants are bothered by the slow build-up of solid HCN polymers. Because of this, purge streams are bled from circulating solutions in different parts of the process. In addition, at intervals, preferably during shut-downs for maintenance, the equipment must be freed from pockets of solid HCN polymer. Should HCN polymers form to an excess degree, not only will the plant be shut down for polymer removal more frequently than is tolerable, but the yield of HCN will fall. Consequently, any process for ammonia separation must be such that the formation of HCN polymers is kept within tolerable limits.